Ion exchange membrane, method for producing ion exchange membrane and electrolyzer

ABSTRACT

An ion exchange membrane, a method for producing an ion exchange membrane, and an electrolyzer that enable a reduction in the electrolytic voltage when subjected to electrolysis are provided which have small influence of impurities in the electrolyte on electrolysis performance, and exert stable electrolysis performance. An ion exchange membrane includes a membrane main body containing a fluorine-containing polymer having an ion exchange group; and a coating layer arranged on at least one face of the membrane main body. The coating layer includes inorganic particles and a binder, a mass ratio of the binder to the total mass of the inorganic particles and the binder in the coating layer is 0.3 or more and 0.9 or less, and a coverage of the membrane main body with the coating layer is 50% or more.

TECHNICAL FIELD

The present invention relates to an ion exchange membrane, a method for producing an ion exchange membrane, and an electrolyzer.

BACKGROUND ART

Fluorine-containing ion exchange membranes, which have excellent heat resistance and chemical resistance, are used as electrolytic diaphragms for alkali chloride electrolysis, ozone generation electrolysis, fuel cells, water electrolysis, and hydrochloric acid electrolysis in various applications, further extending to new applications.

Of these, in alkali chloride electrolysis for producing chlorine and alkali hydroxide, ion exchange membrane process has been predominant recently. Additionally, in order to reduce the electric power consumption rate, natural-circulation zero-gap base electrolyzers including an ion exchange membrane, an anode, and a cathode in close contact one another have become predominant for alkali chloride electrolysis by ion exchange membrane process.

For ion exchange membranes used in alkali chloride electrolysis, required are various capabilities. Among these, in particular, it is required that production efficiency to a current allowed to flow be high from the viewpoint of productivity and the electrolytic voltage be low from the viewpoint of economic efficiency. In alkali chloride electrolysis, in the case where electrolysis at an industrial level is conducted, if the electrolytic voltage can be lowered even slightly and the current efficiency can be raised even slightly, thereby marked energy saving can be achieved.

In alkali chloride electrolysis, it is generally known that gas generated from electrolytic reaction adsorbs onto the surface of the ion exchange membrane to thereby increase the electrolytic voltage. As a countermeasure against this, for example, Patent Literature 1 suggests that gas adsorption onto the surface of an ion exchange membrane is suppressed and the electrolytic voltage is reduced by providing a layer containing a hydrophilic binder and inorganic particles (surface layer) on the surface of the membrane.

In the aqueous alkali chloride solution, impurities such as metals are present. These impurities, if accumulate inside the cation exchange membrane, may cause an increase in the electrolytic voltage, a reduction in the current efficiency, and an increase in the concentration of the impurities in the alkali. Particularly, unlike cation impurities such as Ca and Mg, I is an impurity difficult to reduce even if the electrolyte is treated in advance. Thus, the cation exchange membrane is required to be unlikely to be susceptible to I. As a countermeasure against this, for example, Patent Literature 2 suggests that influence of impurities in the electrolyte on electrolysis performance is small and stable electrolysis performance is obtained by use of specific inorganic particulates as a coating to be applied on the ion exchange membrane.

CITATION LIST Patent Literature

[Patent Literature 1] International Publication No. WO 2015/098769

[Patent Literature 2] Japanese Patent Laid-Open No. 2014-58707

SUMMARY OF INVENTION Technical Problem

In order to hydrophilize a membrane surface to obtain a sufficient effect of preventing adsorption of gas, it is required that a uniform surface layer be formed on the membrane surface while the mass ratio of a binder contained in the surface layer be enhanced based on the total of inorganic particles and the binder.

However, with the method described in Patent Literature 1, in the case where the mass ratio of the binder is enhanced, no uniform surface layer is formed, and the effect of preventing adsorption of gas is reduced.

The ion exchange membrane described in Patent Literature 1 or Patent Literature 2 still has insufficient resistance to impurities, and the stability of the electrolysis performance of the ion exchange membrane to impurities is still not sufficient.

The present invention has been made in view of the above problem, and it is an object of the present invention to provide an ion exchange membrane, a method for producing an ion exchange membrane, and an electrolyzer that enable a reduction in electrolytic voltage by preventing adsorption of gas when subjected to electrolysis and additionally can suppress influence of impurities in the electrolyte on electrolysis performance.

Solution to Problem

As a result of intensive studies to solve the problem, the present inventors have found that a suppression effect on gas adsorption develops and further, influence of impurities in the electrolyte on electrolysis performance can be suppressed by bringing the coverage of the membrane main body with a coating layer to a certain value or more. The present inventors have made further intensive studies based on this result to have found that, even in the case where the binder ratio in a coating solution during formation of a coating layer is raised, the coverage as described above can be obtained by lowering the viscosity of the coating solution, having completed the present invention.

That is to say, the present invention encompasses aspects as follows.

An ion exchange membrane comprising:

a membrane main body comprising a fluorine-containing polymer having an ion exchange group; and

a coating layer arranged on at least one face of the membrane main body;

wherein the coating layer comprises inorganic particles and a binder,

a mass ratio of the binder to a total mass of the inorganic particles and the binder in the coating layer is 0.3 or more and 0.9 or less, and

a coverage of the membrane main body with the coating layer is 50% or more.

The ion exchange membrane according to [1], wherein the inorganic particles are particles comprising at least one inorganic substance selected from the group consisting of oxides of Periodic Table Group IV elements, nitrides of Periodic Table Group IV elements, and carbides of Periodic Table Group IV elements.

The ion exchange membrane according to [1] or [2], wherein the inorganic particles are particles of zirconium oxide.

The ion exchange membrane according to any of [1] to [3], wherein the binder contains a fluorine-containing polymer.

The ion exchange membrane according to any of [1] to [4], wherein the binder contains a fluorine-containing polymer having an ion exchange group derived from a carboxyl group or a sulfo group.

A method for producing the ion exchange membrane according to any of [1] to [5], comprising

spraying a coating solution comprising inorganic particles, a binder, and a solvent by a spray system, followed by drying thereof to form a coating layer on a surface of the membrane main body,

wherein the coating solution has a viscosity of 13 mPa·s or less.

An electrolyzer comprising the ion exchange membrane according to any of [1] to [5].

Advantageous Effects of Invention

According to the present invention, it is possible to provide an ion exchange membrane, a method for producing an ion exchange membrane, and an electrolyzer that enable a reduction in the electrolytic voltage when subjected to electrolysis, additionally, have small influence of impurities in the electrolyte on electrolysis performance, and exert stable electrolysis performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic cross-sectional view showing one embodiment of an ion exchange membrane; and

FIG. 2 illustrates a schematic cross-sectional view showing one embodiment of an electrolyzer.

DESCRIPTION OF EMBODIMENT

Hereinbelow a mode for carrying out the present invention (hereinbelow, referred to simply as “the present embodiment”) will be described in detail. The following present embodiment is by way of illustration for describing the present invention and is not intended to limit the present invention to the following content. The present invention may be modified and implemented as appropriate within the gist thereof.

Note that identical reference numerals are used to denote identical or corresponding components in the description of drawings and the associated description is not repeated. The positional relation such as up and down, left and right, or the like is based upon the positional relation shown in the figures unless otherwise indicated. The dimensional ratios in the drawings are not limited to those illustrated in the drawings. However, the drawings merely illustrate one example of the present embodiment, and the present embodiment is not intended to be construed as being limited thereto.

An ion exchange membrane of the present embodiment is an ion exchange membrane comprising a membrane main body containing a fluorine-containing polymer having an ion exchange group; and a coating layer arranged on at least one face of the membrane main body; wherein the coating layer comprises inorganic particles and a binder, the mass ratio of the binder to the total mass of the inorganic particles and the binder in the coating layer is 0.3 or more and 0.9 or less, and the coverage of the membrane main body with the coating layer is 50% or more. Being thus configured, the ion exchange membrane of the present embodiment enables a reduction in the electrolytic voltage when subjected to electrolysis, additionally has small influence of impurities in the electrolyte on electrolysis performance, and can exert stable electrolysis performance. The ion exchange membrane of the present embodiment and an electrolyzer including the same thus can be preferably used in alkali chloride electrolysis (particularly salt electrolysis).

FIG. 1 is a schematic cross-sectional view showing one embodiment of an ion exchange membrane. An ion exchange membrane 1 of the present embodiment has a membrane main body 10 containing a fluorine-containing polymer having an ion exchange group, and coating layers 11 a and 11 b formed on either side of the membrane main body 10.

As illustrated in FIG. 1, in the ion exchange membrane 1, the membrane main body 10 may comprise a sulfonic acid layer 3 having an ion exchange group derived from a sulfo group (group represented by —SO₃—, hereinafter, referred to also as “sulfonic acid group”) and a carboxylic acid layer 2 having an ion exchange group derived from a carboxyl group (group represented by —CO₂ ⁻, hereinafter, referred to also as “carboxylic acid group”) and furthermore may have strength and dimension stability reinforced by a strengthening material 4. When the ion exchange membrane 1 comprises the sulfonic acid layer 3 and the carboxylic acid layer 2, the membrane tends to develop superior performance as an ion exchange membrane.

The ion exchange membrane of the present embodiment may be not limited to the configuration illustrated in FIG. 1 and may have only either one of a sulfonic acid layer or a carboxylic acid layer. The ion exchange membrane of the present embodiment may not be necessarily reinforced by a strengthening material, and the arranged state of such a strengthening material is not limited to the example of FIG. 1. Additionally, the coating layer may not be necessarily provided on both the faces of the membrane main body and may be provided on only one surface of the membrane main body.

(Membrane Main Body)

First, the membrane main body 10 constituting the ion exchange membrane of the present embodiment 1 will be described.

The membrane main body 10 is only required to have a function of allowing cations to selectively permeate and contain a fluorine-containing polymer having an ion exchange group. The configuration and material thereof are not particularly limited, and various known fluorine-containing polymers may be selected and used as appropriate.

The fluorine-containing polymer having an ion exchange group in the membrane main body 10 can be obtained from a fluorine-containing polymer having an ion exchange group precursor which may become an ion exchange group by hydrolysis or the like. Specifically, the membrane main body 10 can be obtained by preparing a precursor of the membrane main body 10 using, for example, a polymer that has a main chain composed of a fluorinated hydrocarbon and groups that can be converted into ion exchange groups by hydrolysis or the like (ion exchange group precursors) as pendant side chains and is melt-processible (hereinafter, optionally referred to as a “fluorine-containing polymer (a)”) and then converting the ion exchange group precursors into ion exchange groups.

The fluorine-containing polymer (a) can be produced, for example, by copolymerizing at least one monomer selected from the following first group with at least one monomer selected from the following second group and/or the following third group. Alternatively, the polymer (a) can be produced also by homopolymerizing one monomer selected from any of the following first group, the following second group, and the following third group.

Examples of the monomer in the first group include, but not limited to, fluorinated vinyl compounds. Examples of the fluorinated vinyl compound include, but not limited to, vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, and perfluoro(alkyl vinyl ethers). Particularly, when the ion exchange membrane of the present embodiment is used for alkali electrolysis, the fluorinated vinyl compound is preferably a perfluoro monomer. A perfluoro monomer selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, and perfluoro(alkyl vinyl ethers) is preferable.

Examples of the monomer in the second group include, but not limited to, vinyl compounds having functional groups that may be converted to carboxylic acid-type ion exchange groups (carboxylic acid groups). Examples of the vinyl compound having functional groups that may be converted to carboxylic acid groups include, but not limited to, monomers represented by CF₂═CF(OCF₂CYF)₅—O(CZF)_(t)—COOR, wherein s represents an integer of 0 to 2, t represents an integer of 1 to 12, Y and Z each independently represent F or CF₃, and R represents a lower alkyl group. Such a lower alkyl group is, for example, an alkyl group having 1 to 3 carbon atoms.

Of these, compounds represented by CF₂═CF(OCF₂CYF)_(n)—O(CF₂)_(m)—COOR are preferable. Herein, n represents an integer of 0 to 2, m represents an integer of 1 to 4, Y represents F or CF₃, and R represents CH₃, C₂H₅, or C₃H₇.

It should be noted that when the ion exchange membrane according to the present embodiment is used as an ion exchange membrane for alkali electrolysis, it is preferred to use at least a perfluoro compound as a monomer. However, the alkyl group in the ester group (see the above R) is eliminated from the polymer on hydrolysis, and thus, the alkyl group (R) may not be a perfluoro alkyl group in which all the hydrogen atoms have been replaced by fluorine atoms.

As the monomers in the second group, among the above compounds, monomers shown below are more preferred:

CF₂═CFOCF₂—CF(CF₃)OCF₂COOCH₃, CF₂═CFOCF₂CF(CF₃)O(CF₂)₂COOCH₃, CF₂═CF[OCF₂—CF(CF₃)]₂O(CF₂)₂COOCH₃, CF₂═CFOCF₂CF(CF₃)O(CF₂) COOCH₃, CF₂═CFO(CF₂)₂COOCH₃, and CF₂═CFO(CF₂)₃COOCH₃.

Examples of monomers in the third group include vinyl compounds having functional groups that may be converted to sulfone-type ion exchange groups (sulfonic acid groups). As the vinyl compound having functional groups that may be converted to sulfonic acid groups, for example, monomers represented by CF₂═CFO—X—CF₂—SO₂F are preferable, wherein X represents a perfluoroalkylene group. Specific examples thereof include the monomers shown below:

CF₂═CFOCF₂CF₂SO₂F, CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F, CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CF₂SO₂F, CF₂═CF(CF₂)₂SO₂F, CF₂═CFO[CF₂CF(CF₃)O]₂CF₂CF₂SO₂F, and CF₂═CFOCF₂CF(CF₂OCF₃)OCF₂CF₂SO₂F.

Of these, CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CF₂SO₂F and CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F are more preferred.

Copolymers that can be obtained from these monomers can be produced by a polymerization method developed for homopolymerization and copolymerization of fluorinated ethylene, particularly, a common polymerization method that is used for tetrafluoroethylene. For example, in a non-aqueous method, a polymerization reaction can be carried out by using an inert solvent such as perfluorohydrocarbons and chlorofluorocarbons in the presence of a radical polymerization initiator such as perfluorocarbon peroxides and azo compounds and under conditions of a temperature of 0 to 200° C. and a pressure of 0.1 to 20 MPa.

In the above copolymerization, the type and ratio of the above monomers are not particularly limited and can be selected and determined depending on the type and amount of the functional group that is desired to be imparted to a fluorine-containing polymer to be obtained. For example, in the case of a fluorine-containing polymer containing only carboxylic acid groups, it is only required to select at least one monomer from each of the first group and the second group described above and copolymerize the monomers. Alternatively, in the case of a fluorine-containing polymer containing only sulfonic acid groups, it is only required to select at least one monomer from each of the first group and the third group described above and copolymerize the monomers. Furthermore, in the case of a fluorine-containing polymer containing carboxylic acid groups and sulfonic acid groups, it is only required to select at least one monomer from each of the first group, the second group, and the third group described above and copolymerize the monomers. In this case, an intended fluorine-containing polymer can be obtained also by separately producing a copolymer composed of the first group and the second group described above and a copolymer composed of the first group and the third group described above and then mixing the copolymers. The ratio of each monomer to be mixed is not particularly limited. In order to increase the amount of functional groups per unit polymer, it is only required to increase the proportion of monomers selected from the second group and the third group described above.

The total ion exchange capacity of the fluorine-containing polymer is not particularly limited, and is preferably 0.5 mg equivalent/g or more and 2.0 mg equivalent/g or less, more preferably 0.6 mg equivalent/g or more and 1.5 mg equivalent/g or less. The total ion exchange capacity herein refers to the equivalents of exchange groups per unit weight of a dry resin and can be determined by neutralization titration or the like.

In the membrane main body 10 of the ion exchange membrane 1 illustrated in FIG. 1, the sulfonic acid layer 3 containing a fluorine-containing polymer having a sulfonic acid group and the carboxylic acid layer 2 containing a fluorine-containing polymer having a carboxylic acid group are laminated. In the case of the membrane main body 10 having such a layer structure, the selective permeability of cations such as sodium ions tends to be further improved.

When the ion exchange membrane 1 illustrated in FIG. 1 is arranged in an electrolyzer, the membrane 1 is usually arranged such that the sulfonic acid layer 3 is located on the anode side of the electrolyzer and the carboxylic acid layer 2 is located on the cathode side of the electrolyzer.

The sulfonic acid layer 3 is preferably constituted by a material having low electrical resistance, and preferably has a membrane thickness larger than that of the carboxylic acid layer 2 from the viewpoint of membrane strength. The membrane thickness of the sulfonic acid layer 3 is preferably twice or more and 25 times or less that of carboxylic acid layer 2, more preferably three times or more and 15 times or less.

The carboxylic acid layer 2 preferably has a high anion elimination property even if having a small membrane thickness. The anion elimination property referred to herein is a property of preventing intrusion and permeation of anion to the ion exchange membrane 1. In order to improve the anion elimination property, it is effective to provide a carboxylic acid layer having a smaller ion exchange capacity on the sulfonic acid layer or the like.

As a fluorine-containing polymer for use in the sulfonic acid layer 3, it is preferable to use a polymer obtained by using, for example, CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F as a monomer in the third group.

As a fluorine-containing polymer for use in the carboxylic acid layer 2, it is preferable to use a polymer obtained by using, for example, CF₂═CFOCF₂CF(CF₂)O(CF₂)₂COOCH₃ as a monomer in the second group.

(Coating Layer)

The ion exchange membrane of the present embodiment has a coating layer provided on at least one face of the membrane main body. Further, in the ion exchange membrane 1 illustrated in FIG. 1, coating layers 11 a and 11 b are formed on either face of the membrane main body 10.

The coating layer in the present embodiment contains inorganic particles and a binder, and the coverage of the membrane main body with the coating layer is 50% or more. The coverage described above herein is a value to be calculated by a measurement method described in examples mentioned below. In the present embodiment, when the coverage described above is sufficiently high, it is possible to suppress gas adsorption onto the ion exchange membrane during electrolysis, and as a result, it is possible to sufficiently reduce the electrolytic voltage. From the similar viewpoint, the coverage described above is more preferably 60% or more, even more preferably 65% or more. A higher coverage is more preferable, and from such a viewpoint, the coverage can be 100%.

A specific method for measuring the coverage is as follows.

An ion exchange membrane having a coating layer is observed from the coating layer side using a microscope (manufactured by KEYENCE CORPORATION, VHX-6000, magnification: 500 times). The coated portion is observed to have high lightness due to scattering of light caused by the inorganic particles and the binder. Thus, the region having a lightness of 150 or more in the observed image is taken as the coated portion, the region having a lightness of less than 150 is taken as the non-coated portion, and binarization processing is conducted. The proportion of the coated portion, when the entire observed image is taken as 100, is calculated as a coverage.

The coverage is information on a coating layer to be obtained in a visual field range (0.7×0.5 mm) of the microscope mentioned above. In contrast, the distribution density of the coating layer mentioned below is information on the coating layer obtained in an X-ray fluorescence measuring range (10×10 mm). In this manner, information on a more micro region can be obtained by the coverage described above than by the distribution density.

The coverage of the coating layer in the present embodiment is not limited to the following. For example, as mentioned below, sufficiently reducing the viscosity of a coating solution during the spraying of the coating solution by spraying enables the coverage to be adjusted in the range described above.

The average particle size of the inorganic particles in the present embodiment is not particularly limited and is preferably 0.90 μm or more. When the average particle size of the inorganic particles is 0.90 μm or more, the durability to impurities tends to be further improved. In the present embodiment, irregular inorganic particles are preferably used, and inorganic particles obtained by pulverizing raw stones are more preferably used.

The average particle size of the inorganic particles also can be 2 μm or less. When the average particle size of the inorganic particles is 2 μm or less, it is more likely to be able to further prevent damage in the membrane caused by the inorganic particles. The average particle size of the inorganic particles is more preferably 0.90 μm or more and 1.2 μm or less. The average particle size is still more preferably 1 μm or more and 1.2 μm or less.

In the present description, the average particle size means a median diameter (D50) and can be measured with a particle size analyzer (“SALD2200”, SHIMADZU CORPORATION).

The inorganic particles in the present embodiment are preferably hydrophilic. Hydrophilicity refers a property of a solid surface to easily moisten with water. Generally, those having a small contact angle can be evaluated as hydrophilic. For example, inorganic particles having a contact angle of the order of 90° can be evaluated as hydrophilic. The contact angle is preferably 90° or less, more preferably 40° or less. The contact angle herein means an angle formed by the tangent line of the liquid surface at a contact between a solid and a liquid and the solid surface. A contact angle meter (“DMo-601” manufactured by Kyowa Interface Science Co., Ltd.) can be used to bring a droplet into contact with a solid surface and analyze the image of the droplet on contact to thereby calculate the contact angle. When the inorganic particles are hydrophilic, aligning the particles to the surface of the coating layer tends to enable gas adsorption to the ion exchange membrane during electrolysis to be more suppressed. The inorganic particles more preferably contain at least one inorganic substance selected from the group consisting of oxides of Periodic Table Group IV elements, nitrides of Periodic Table Group IV elements, and carbides of Periodic Table Group IV elements. Specific examples thereof include, but not limited to, zirconium oxide, silica oxide, tin oxide, titanium oxide, nickel oxide, SiC, and ZrC. From the viewpoint of durability, particles of zirconium oxide are further preferable.

The inorganic particles in the present embodiment are preferably inorganic particles produced by pulverizing raw stones of the inorganic particles. Note that inorganic particles are produced by melting and purifying the raw stones of the inorganic particles and spherical particles having a uniform particle size can be also used as inorganic particles in the coating layer.

Examples of the pulverizing method include, but not particularly limited to, a ball mill, beads mill, colloid mill, conical mill, disc mill, edge mill, grain mill, hammer mill, pellet mill, VSI mill, Wiley mill, roller mill, and jet mill. The inorganic particles after pulverization are preferably washed, and, as the washing method at this time, acid treatment is preferable. This treatment can reduce impurities such as iron attached to the surface of the inorganic particles.

In the present embodiment, the coating layer contains a binder. The binder is a component that retains the inorganic particles on the surface of the ion exchange membrane to form a coating layer. The binder preferably contains a fluorine-containing polymer, from the viewpoint of the resistance to a electrolyte and products from electrolysis. As a fluorine-containing polymer to be contained as the binder in the coating layer, a polymer of the same type as the fluorine-containing polymer constituting the membrane main body may be used, or a polymer of a different type may be used. In addition to such fluorine-containing polymers, as the binder component in the coating layer, various known compounds may be employed, but the content of the fluorine-containing polymer in the binder is preferably 90% by mass or more.

The binder in the present embodiment is more preferably a fluorine-containing polymer having carboxylic acid groups or sulfonic acid groups, from the viewpoint of the resistance to a electrolyte and products from electrolysis and adhesion to the surface of the ion exchange membrane. When a coating layer is provided on a layer containing a fluorine-containing polymer having sulfonic acid groups (sulfonic acid layer), a fluorine-containing polymer having sulfonic acid groups is further preferably used as the binder in the coating layer. Alternatively, when a coating layer is provided on a layer containing a fluorine-containing polymer having carboxylic acid groups (carboxylic acid layer), a fluorine-containing polymer having carboxylic acid groups is further preferably used as the binder in the coating layer.

In the present embodiment, the mass ratio of the binder to the total mass of the inorganic particles and the binder in the coating layer is 0.3 or more and 0.9 or less. The present inventors have found that an increase in the mass ratio of the binder described above in the coating layer reduces the ion permeability resistance of the ion exchange membrane in itself. In other words, the ion permeability resistance of the ion exchange membrane in itself is further reduced by making the mass ratio of the binder 0.3 or more. Thus, this reduction, in combination with increasing the coverage of the coating layer as mentioned above, can markedly reduce the electrolytic voltage. Also, when the mass ratio of the binder is 0.9 or less, the effect of preventing adsorption of gas due to the inorganic particles can be obtained, and thus, it is possible to reduce the electrolytic voltage. From the similar viewpoint, the mass ratio of the binder is preferably 0.32 or more and 0.9 or less, more preferably 0.35 or more and 0.9 or less, still more preferably 0.4 or more and 0.9 or less.

The distribution density of the coating layer in the ion exchange membrane is not particularly limited, and is preferably 0.05 mg or more and 2 mg or less per 1 cm², more preferably 0.5 mg or more and 2 mg or less per 1 cm². The distribution density described above can be measured by a method described in examples mentioned below. Additionally, the distribution density described above can be adjusted within the range described above by, for example, changing the amount to be discharged during the spraying for application or changing the number of recoating.

(Strengthening Material)

The ion exchange membrane of the present embodiment preferably has a strengthening material arranged inside the membrane main body.

In the present embodiment, the strengthening material functions as at least one of reinforcement yarn and sacrifice yarn. Examples thereof include, but not limited to, fabric formed by weaving reinforcement yarn and sacrifice yarn. Disposing the strengthening material inside the membrane main body enables, in particular, expansion and contraction of the ion exchange membrane to be controlled within a desired range. Such an ion exchange membrane does not expand and contract more than required on electrolysis and the like and can maintain excellent dimension stability for a long period.

The configuration of the strengthening material is not particularly limited, and the strengthening material may be formed by, for example, spinning yarn called reinforcement yarn. The reinforcement yarn herein referred to is a member that constitutes the strengthening material, the member being yarn that can impart desired dimension stability and mechanical strength to the ion exchange membrane and additionally can be present stably in the ion exchange membrane. Use of a strengthening material formed by spinning such reinforcement yarn can impart further excellent dimension stability and mechanical strength to the ion exchange membrane.

The strengthening material and materials of reinforcement yarn used therefor are not particularly limited and are preferably materials resistant to acid and alkali and the like. From the viewpoint of heat resistance and chemical resistance over a long term, fibers constituted by a fluorine-containing polymer are preferable.

As the fluorine-containing polymer to be used in the strengthening material, those used in the membrane main body mentioned above can be used as well. Examples thereof particularly include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoro alkyl vinyl ether copolymers (PFA), tetrafluoroethylene-ethylene copolymers (ETFE), tetrafluoroethylene-hexafluoropropylene copolymers, trifluorochlorethylene-ethylene copolymers, and vinylidene fluoride polymers (PVDF). Of these, particularly from the viewpoint of heat resistance and chemical resistance, fibers constituted by polytetrafluoroethylene are preferably employed.

The yarn diameter of the reinforcement yarn to be used in the strengthening material is not particularly limited and preferably 20 to 300 deniers, more preferably 50 to 250 deniers. The weaving density (number of strands of yarn inserted per unit length) is preferably 5 to 50 strands/inch. The form of the strengthening material is not particularly limited, and woven fabric, non-woven fabric, knitted fabric or the like is used, for example. However, the form of woven fabric is preferable. Woven fabric having a thickness of preferably 30 to 250 μm, more preferably 30 to 150 μm is used.

As the woven fabric or knitted fabric, monofilaments, multifilaments, or yarn and slit yarn thereof or the like can be used. Various weaving methods can be used, such as plain weave, leno weave, knitted weave, cord weave, and seersucker.

The weaving method and arrangement for the strengthening material in the membrane main body are not particularly limited. An appropriately and suitably arrangement can be employed in consideration of the size and shape of the ion exchange membrane, physical properties required from the ion exchange membrane, an environment of usage and the like.

For example, the strengthening material may be arranged along a predetermined direction of the membrane main body. From the viewpoint of the dimension stability, it is preferred that a strand of the strengthening material be arranged along a predetermined first direction and another strand of the strengthening material be arranged along a second direction substantially perpendicular to the first direction. A plurality of strands of the strengthening material is arranged inside the longitudinal-direction membrane main body so as to substantially directly run. This can impart further excellent dimension stability and mechanical strength in many directions. For example, an arrangement is preferred in which the strengthening material arranged along the longitudinal direction (warps) is interwoven with the reinforcement yarn arranged along the lateral direction (wefts) on the surface of the membrane main body. The arrangement is more preferably in the form of plain weave woven by allowing warps and wefts to run over and under each other alternately, leno weave in which two warps are woven into wefts while twisted, basket weave woven by inserting, into two or more parallelly-arranged warps, wefts of the same number, or the like, from the viewpoint of dimension stability and mechanical strength.

Particularly, the strengthening material is preferably arranged along both the machine direction (MD) and the transverse direction (TD) of the ion exchange membrane. That is, the reinforcement yarn is preferably plain-woven in the MD and the TD. The MD herein refers to the direction in which the membrane main body and strengthening material are carried (flow direction) in the production step of ion exchange membrane described below, and the TD refers to the direction substantially perpendicular to the MD. Yarn woven along the MD is referred to as MD yarn, and yarn woven along the TD is referred to as TD yarn. The ion exchange membrane used in electrolysis is usually rectangular. Thus, frequently, its longitudinal direction is the MD, and the width direction is the TD. By interweaving the strengthening material as MD yarn into the strengthening material as TD yarn, it is possible to impart further excellent dimension stability and mechanical strength in many directions.

The arrangement interval for the strengthening material is not particularly limited. The strengthening material can be appropriately and suitably arranged in consideration of physical properties required from the ion exchange membrane, an environment of usage and the like.

Of strengthening materials, particularly preferred forms are tape yarn and highly-oriented monofilaments containing PTFE from the viewpoint of chemical resistance and heat resistance. Specifically, the strengthening material is more preferably a strengthening material formed by plain-weaving using 50 to 300 deniers of tape yarn obtained by slitting a high-strength porous sheet made of PTFE into a tape form or a highly-oriented monofilament made of PTFE at a weaving density of 10 to 50 strands/inch and having a thickness in the range of 50 to 100 μm. The aperture ratio of the ion exchange membrane including such strengthening material is further preferably 60% or more.

Examples of the shape of the reinforcement yarn include round yarn and tape yarn. Preferably, the yarn is round yarn.

(Continuous Hole)

The ion exchange membrane of the present embodiment preferably has continuous holes inside the membrane main body.

The continuous holes are holes that may serve as a flow path for cations generated during electrolysis and a electrolyte. Additionally, continuous holes, which are tubular holes formed inside the membrane main body, are formed by dissolution of the strengthening material (sacrifice yarn) mentioned below. The shape, diameter, and the like of the continuous holes can be controlled by selecting the shape and diameter of the strengthening material (sacrifice yarn).

Forming continuous holes in the ion exchange membrane can ensure the mobility of alkali ions generated during electrolysis and a electrolyte. The shape of the continuous holes is not particularly limited, but, according to the production method described below, may be the shape of the strengthening material (sacrifice yarn) to be used for formation of the continuous holes.

In the present embodiment, the continuous holes are preferably formed so as to alternately penetrate the anode side of the strengthening material (sulfonic acid layer side) and through the cathode side (carboxylic acid layer side). Such a structure enables cations (e.g., sodium ions) transferred through the electrolyte filling the continuous holes to flow also to the cathode side of strengthening material, in a portion in which continuous holes are formed on the cathode side of the strengthening material. As a result, the flow of the cations is not interrupted, and thus, it is possible to further reduce the electrical resistance of the ion exchange membrane.

The continuous holes may be formed along only one predetermined direction of the membrane main body constituting the ion exchange membrane of the present embodiment, but, from the viewpoint of exerting more stable electrolytic performance, the continuous holes are preferably formed both in the longitudinal direction and the lateral direction of the membrane main body.

[Production Method]

The method of producing an ion exchange membrane according to the present embodiment is not particularly limited as long as an ion exchange membrane having the configuration mentioned above can be obtained, and the ion exchange membrane is preferably produced by a method having the following step (1) to step (6):

step (1): a step of producing a fluorine-containing polymer having an ion exchange group or an ion exchange group precursor which may become an ion exchange group by hydrolysis;

step (2): as required, a step of obtaining a strengthening material in which sacrifice yarn, which is soluble in acid or alkali and forms continuous holes, is arranged between adjacent strands of reinforcement yarn by interweaving at least a plurality of strands of the reinforcement yarn and the sacrifice yarn;

step (3): a step of forming a film from the fluorine-containing polymer having an ion exchange group or an ion exchange group precursor which may become an ion exchange group by hydrolysis;

step (4): a step of embedding the strengthening material in the film to obtain a membrane main body including the strengthening material arranged therein, as required;

step (5): a step of hydrolyzing the membrane main body obtained in the step (4) (hydrolysis step); and

step (6): a step of providing a coating layer on the membrane main body obtained in the step (5) (coating step).

The method for producing an ion exchange membrane of the present embodiment is mainly characterized by adjusting the viscosity of the coating solution in the coating step (6). Hereinbelow, each of the steps will be described in detail.

Step (1): Step of Producing Fluorine-Containing Polymer

In the step (1), starting material monomers described in the first group to the third group described above are used to produce a fluorine-containing polymer. In order to control the ion exchange capacity of the fluorine-containing polymer, it is only required that the mixture ratio of the starting material monomers be adjusted in production of the fluorine-containing polymer constituting each layer.

Step (2): Step of Producing Strengthening Material

A strengthening material is woven fabric obtained by weaving reinforcement yarn, and the like. Embedding the strengthening material in the membrane can provide a membrane main body including the strengthening material therein.

Step (3): Step of Film Formation

In the step (3), a film is formed from the fluorine-containing polymer obtained in the step (1) by use of an extruder. The film may have a single-layer structure, a two-layer structure of a sulfonic acid layer and a carboxylic acid layer as mentioned above, or a multilayer structure of three or more layers.

Step (4): Step of Obtaining Membrane Main Body

In the step (4), the strengthening material obtained in the step (2) is embedded in the film obtained in the step (3) to obtain a membrane main body including the strengthening material therein.

Examples of a method for forming a membrane main body include (i) a method in which a fluorine-containing polymer having carboxylic acid group precursors located on the cathode side (e.g., carboxylate functional groups) (hereinafter, a layer constituted by the polymer is referred to as a first layer) and a fluorine-containing polymer having sulfonic acid group precursors (e.g., sulfonyl fluoride functional groups) (hereinafter, a layer constituted by the polymer is referred to as a second layer) are used to form a film by a coextrusion process, a strengthening material and the second layer/first layer composite film are laminated in this order, via a breathable heat-resistant release paper, on a flat plate or drum having many micropores on the surface thereof and, using a heat source and a vacuum source used as required, are integrated at a temperature at which each of the polymers melts while the air among each of the layers was evacuated by reduced pressure; and (ii) a method in which, separately from the second layer/first layer composite film, a fluorine-containing polymer having sulfonic acid group precursors (third layer) is used singly to form a film in advance, the third layer film, a strengthening material, and the composite film constituted by the second layer/first layer are laminated in this order, via a breathable heat-resistant release paper, on a flat plate or drum having many micropores on the surface thereof and, using a heat source and a vacuum source used as required, are integrated at a temperature at which each of the polymers melts while the air among each of the layers was evacuated by reduced pressure.

Coextruding the first layer and the second layer herein is preferable because of its contribution to an increase in the adhesive strength in the interface.

The method including integration under reduced pressure is preferable because the thickness of the third layer on the strengthening material tends to be larger than that of a pressure-application press method. Furthermore, the mechanical strength of the ion exchange membrane tends to be able to be sufficiently maintained because the strengthening material is fixed on the inner surface of the membrane main body.

A variety of laminations described herein is exemplary. After an appropriate and suitable lamination pattern (for example, combination of each of layers) is selected in consideration of the layer configuration and physical properties of a desired membrane main body and the like, coextrusion can be carried out.

For the purpose of further improving the electric properties of the ion exchange membrane, it is also possible to additionally interpose a fourth layer constituted by a fluorine-containing polymer having both carboxylic acid group precursors and sulfonic acid group precursors between the first layer and the second layer, or to use a fourth layer constituted by a fluorine-containing polymer having both carboxylic acid group precursors and sulfonic acid group precursors instead of the second layer.

The method for forming the fourth layer may be a method in which a fluorine-containing polymer having carboxylic acid group precursors and a fluorine-containing polymer having sulfonic acid group precursors are separately produced and then mixed, or may be a method in which a copolymer produced from a monomer having carboxylic acid group precursors and a monomer having sulfonic acid group precursors is used.

When the fourth layer is used as a component of the ion exchange membrane, a coextruded film of the first layer and the fourth layer is formed, the third layer and the second layer are each used separately from this to form a film singly, and the films may be laminated in the manner mentioned above. Alternatively, three layers of the first layer/fourth layer/second layer may be coextruded at once to form a film.

In this case, the direction in which the extruded film flows is the MD. In this manner, the membrane main body comprising a fluorine-containing polymer having an ion exchange group can be formed on the strengthening material.

Additionally, the ion exchange membrane of the present embodiment preferably has protruded portions constituted by the fluorine-containing polymer having sulfonic acid groups, that is, raised portions, on the surface side composed of the sulfonic acid layer. As a method for forming such raised portions on the surface of the membrane main body, which is not particularly limited, a known method can be employed including forming raised portions on a resin surface. Specifically, an example is a method including subjecting the surface of the membrane main body to embossing. For example, when the composite film, strengthening material and the like are integrated, the raised portions described above can be formed using release paper embossed in advance. In the case where raised portions are formed by embossing, the height and arrangement density of the raised portions can be controlled by controlling the emboss shape to be transferred (shape of the release paper).

(5) Hydrolysis Step

In the step (5), a step of hydrolyzing the membrane main body obtained in the step (4) to thereby convert the ion exchange group precursor into an ion exchange group (hydrolysis step) is performed.

Additionally, in the step (5), it is possible to form dissolution holes in the membrane main body by dissolving and removing the sacrifice yarn included in the membrane main body with acid or alkali. The sacrifice yarn may remain in the continuous holes, not completely dissolved and removed. Alternatively, the sacrifice yarn remaining in the continuous holes may be dissolved and removed by the electrolyte when the ion exchange membrane is subjected to electrolysis.

The sacrifice yarn has solubility in acid or alkali in the step of producing an ion exchange membrane or under an electrolysis environment. Dissolution of the sacrifice yarn allows continuous holes to be formed at corresponding sites.

(6) Coating Step

In the step (6), a coating solution containing inorganic particles, a binder, and a solvent is sprayed by a spray system and dried to thereby form a coating layer on the surface of the membrane main body.

In the present embodiment, making the viscosity of the coating solution sufficiently small makes the coating solution easily wet and spread on the surface of the membrane main body when the coating solution is sprayed, and a coating layer to be formed is uniformly formed on the surface of the membrane main body. In this manner, making the viscosity of the coating solution sufficiently small can make the coverage of the coating layer sufficiently high, despite of use of the coating solution having a high binder proportion.

As the inorganic particles, those obtained by pulverizing raw stones can be preferably used. As the binder, binders obtained by hydrolyzing a fluorine-containing polymer having ion exchange group precursors with an aqueous solution containing dimethyl sulfoxide (DMSO) and potassium hydroxide (KOH) and then immersing the polymer in hydrochloric acid to substitute the counter ions of the ion exchange groups with H+(e.g., a fluorine-containing polymer having carboxylic groups or sulfo groups) can be preferably used. Such binders are preferable because of being more likely to dissolve in water or ethanol mentioned below.

This binder is preferably dissolved in a solution prepared by mixing water and ethanol, for example. Note that a volume ratio between water and ethanol is preferably 10:1 to 1:10, more preferably 5:1 to 1:5, still more preferably 2:1 to 1:2.

The inorganic particles are dispersed with a ball mill into the dissolving liquid thus obtained to thereby obtain a coating solution. In this time, adjusting the time and rotational speed during dispersion also enables the average particle size of the inorganic particles and the viscosity of the coating solution to be adjusted.

A preferable amount of the inorganic particles and the binder blended is 0.3 or more and 0.9 or less as the binder ratio to the total mass of the inorganic particles and the binder in the coating solution described above, from the viewpoint of further reducing ion permeability resistance of the ion exchange membrane in itself. Since the mass ratio of the binder in the coating solution as the feed ratio described above corresponds to the binder proportion after the coating layer is formed, the binder proportion in the coating layer in the ion exchange membrane can be identified from the feed ratio.

When the inorganic particles are dispersed, a surfactant may be added to the dispersion liquid. The surfactant is preferably a nonionic surfactant, and examples thereof include, but not limited to, HS-210, NS-210, P-210, and E-212 manufactured by NOF CORPORATION.

The viscosity of the coating solution thus obtained is preferably 13 mPa-s or less, more preferably 11 mPa-s or less. When the viscosity is low, the coating solution uniformly wets and spreads on the membrane surface to thereby allow the effect of preventing adsorption of gas to be sufficiently exerted.

For control of the viscosity, a known method can be employed. Examples of the method include change of various conditions during dissolution of the binder polymer, change of various conditions during dispersion of the inorganic particles, and addition of the surfactant and the viscosity adjuster mentioned above. The viscosity of the coating solution described above can be measured by a method described in examples.

Application of the coating solution obtained on at least one surface of the membrane main body by spray application forms a coating layer, and thus, the ion exchange membrane of the present embodiment can be obtained.

As described above, the method for producing an ion exchange membrane of the present embodiment comprises spraying a coating solution comprising inorganic particles, a binder, and a solvent by a spray system and drying the liquid to form a coating layer on a surface of the membrane main body, and the viscosity of the coating solution is preferably 13 mPa-s or less.

[Electrolyzer]

The ion exchange membrane of the present embodiment can be used as a constituting member of an electrolyzer. That is, the electrolyzer of the present embodiment comprises the ion exchange membrane of the present embodiment. FIG. 2 is a schematic view of one embodiment of an electrolyzer according to the present embodiment.

An electrolyzer 100 of the present embodiment includes at least an anode 200, a cathode 300, and an ion exchange membrane 1 of the present embodiment arranged between the anode 200 and the cathode 300. The electrolyzer 100 comprising the ion exchange membrane 1 described above is described herein by way of example. However, the present invention is not limited thereto and may be implemented by variously modifying the configuration thereof within the effect of the present embodiment.

The electrolyzer 100 can be used for various types of electrolysis, and as a typical example, a case when the electrolyzer is used for electrolysis of an alkali chloride aqueous solution will be described below.

Electrolysis conditions are not particularly limited, and the electrolysis can be carried out under known conditions. For example, with the anode chamber provided with a 2.5 to 5.5 N alkali chloride aqueous solution and the cathode chamber provided with water or diluted alkali hydroxide aqueous solution, electrolysis is carried out with a direct current.

The configuration of the electrolyzer according to the present embodiment is not particularly limited and may be monopolar or bipolar, for example. Materials constituting the electrolyzer 100 are not particularly limited. As materials for the anode chamber, titanium and the like, which are resistant to alkali chloride and chlorine, are preferable. As materials for the cathode chamber, nickel and the like, which are resistant to alkali hydroxide and hydrogen, are preferable. As for the arrangement of the electrodes, even when the ion exchange membrane 1 and the anode 200 are arranged with an appropriate gap therebetween or even when the anode 200 is arranged in contact with the ion exchange membrane 1, the ion exchange membrane 1 can be used without any problem. The cathode is generally arranged with an appropriate gap from the ion exchange membrane, but a contact-type electrolyzer having such a gap (zero-gap base electrolyzer) may be used without any problem.

EXAMPLES

Hereinafter, the present embodiment will be described further in detail based on examples. The present embodiment is not intended to be limited only to these examples.

(Viscosity Measurement)

An E-type viscometer (TV-35L, manufactured by Toki Sangyo Co., Ltd., standard cone rotor) was used to measure the viscosity of the coating solution at a temperature of 25° C. and a number of revolutions of 10 rpm.

(Coverage)

An ion exchange membrane was observed from the coating layer side using a microscope (manufactured by KEYENCE CORPORATION, VHX-6000, magnification: 500 times). The coated portion was observed to have high lightness due to scattering of light caused by the inorganic particles and the binder. Thus, the region having a lightness of 150 or more of the observed image was taken as the coated portion, the region having a lightness of less than 150 was taken as the non-coated portion, and binarization processing was conducted. The proportion of the coated portion, when the entire observed image was taken as 100, was calculated as a coverage.

(Distribution Density of Coating Layer)

Zirconium present in the dried coating layer was quantified by an X-ray fluorescence analyzer (X-MET 8000, manufactured by HORIBA, Ltd.), the amount was converted to the entire weight of the coating layer containing the binder by means of a calibration curve prepared in advance, and the distribution density was calculated. A sample of which distribution density was known by means of gravimetry was used for preparation of the calibration curve, and such a calibration curve was prepared for each of coating solutions having a different binder proportion.

[Electrolysis Evaluation]

The electrolyzer for use in electrolysis was one in which four natural-circulation zero-gap electrolysis cells were arranged in series, each of which had a structure including an ion exchange membrane arranged between an anode and a cathode. As the cathode, woven mesh was used formed by knitting nickel fine wire having a diameter of 0.15 mm and coated with cerium oxide and ruthenium oxide as catalysts in a sieve mesh size of 50. To bring the cathode into close contact with the ion exchange membrane, a mat formed by knitting nickel fine wire was arranged between a collector made of nickel expanded metal and the cathode. As the anode, used was titanium expanded metal coated with ruthenium oxide, iridium oxide, and titanium oxide as catalysts.

Brine was supplied to the anode side while the concentration was adjusted to be 205 g/L by use of the electrolyzer described above, and water was supplied to the cathode side while the sodium hydroxide concentration was maintained at 32% by mass. The temperature of the electrolyzer was set to 85° C., and electrolysis was performed under conditions where the current density was 6 kA/m², and the fluid pressure on the cathode side of the electrolyzer was higher than the fluid pressure on the anode side by 5.3 kPa. The interelectrode voltage between the anode and the cathode of the electrolyzer was measured everyday using TR-V1000, a voltmeter manufactured by KEYENCE CORPORATION, and the average value for 7 days was determined as the electrolytic voltage.

[Test on Resistance to Impurities]

Brine was supplied to the anode side while the concentration was adjusted to be 205 g/L by use of the electrolyzer described above, and water was supplied to the cathode side while the sodium hydroxide concentration was maintained at 32% by mass. Then, brine containing 10 ppm of I and 0.03 ppm of Ba as impurities was used, the temperature of the electrolyzer was set to 85° C., and electrolysis was performed for 9 days under conditions where the current density was 6 kA/m² and the fluid pressure on the cathode side of the electrolyzer was higher than the fluid pressure on the anode side by 5.3 kPa. Thereafter, the increase or decrease in the value of the current efficiency on the 9th day after the electrolysis was measured from the value of the current efficiency on the 1st day of the electrolysis to determine the change ratio per day.

This test is an acceleration test in which impurities were added at a larger excess concentration than the concentration of impurities acceptable under normal electrolysis conditions. When the decrease in the current efficiency was 0.75 h/day or less in this test, the electrolyzer was considered to cause no decrease in the current efficiency due to influence of impurities under normal electrolysis conditions. In addition, when the decrease in the current efficiency was 0.55%/day or less in this test, the electrolyzer was considered to be capable of suppressing the decrease in the current efficiency due to influence of the primary increase in the concentration of the impurities, which was caused by a problem in the salt water purification system or the like.

Example 1

As reinforcement yarn, a yarn-like material prepared by twisting 100-denier tape yarn made of polytetrafluoroethylene (PTFE) at 900 turns/m (hereinafter, referred to as PTFE yarn) was used. As warp sacrifice yarn, yarn prepared by twisting polyethylene terephthalate (PET) of 35 deniers and 8 filaments at 200 turns/m (hereinafter, referred to as PET yarn) was used. As weft sacrifice yarn, yarn prepared by twisting polyethylene terephthalate (PET) of 35 deniers and 8 filaments at 200 turns/m (hereinafter, referred to as PET yarn) was used. First, plain-weaving was carried out with the PTFE yarn arranged at 24 strands/inch and two strands of the sacrifice yarn arranged between adjacent strands of the PTFE yarns to thereby obtain woven fabric having a thickness of 100 μm.

Then, provided were a polymer (A1) as a dried resin, which was a copolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)OCF₂CF₂COOCH₃ and had an ion exchange capacity of 0.85 mg equivalent/g and a polymer (B1) as a dried resin, which was a copolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F and had an ion exchange capacity of 1.03 mg equivalent/g. These polymers (A1) and (B1) were used to obtain a two-layer film X having a thickness of the polymer (A1) layer of 20 μm and a thickness of the polymer (B1) layer of 94 μm, by a coextrusion T-die method. Note that the ion exchange capacity of each polymer indicates the ion exchange capacity when the ion exchange group precursors of each polymer were converted by hydrolysis into ion exchange groups.

A polymer was obtained, which was a copolymer of CF₂═CF₂ and CF₂═CFO—CF₂CF(CF₃)O—(CF₂)₂—SO₂F and had an ion exchange capacity of 1.05 mg equivalent/g. This polymer was extruded through a single-layer T die to obtain a single-layer film Y having a thickness of 20 μm.

Subsequently, on a drum including a heat source and a vacuum source therein and having many micropores on the surface thereof, release paper embossed in advance, the film Y, a strengthening material (woven fabric obtained above), and the film X were laminated in the order mentioned and heated under reduced pressure for two minutes, under conditions of a drum temperature of 240° C. and a degree of reduced pressure of 0.067 MPa. Then, release paper was removed to thereby obtain a composite membrane having asperities. The composite membrane obtained was saponified by immersion in an aqueous solution at 90° C. containing dimethyl sulfoxide (DMSO) of 30% by mass and potassium hydroxide (KOH) of 15% by mass for an hour. Thereafter, the membrane was immersed in 0.5N NaOH at 90° C. for an hour to substitute ions attached to the ion exchange groups with Na and then washed with water. Further, the membrane was dried at 60° C. to thereby obtain a membrane main body.

Additionally, a polymer (B3) as a dried resin, which was a copolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F and had an ion exchange capacity of 1.05 mg equivalent/g was hydrolyzed and then converted into its acid form with hydrochloric acid. In a solution obtained by dissolving this acidic-form polymer (B3) at a proportion of 5% by mass in a 50/50 (mass ratio) mixed solution of water and ethanol, zirconium oxide particles having a primary particle size of 1.15 μm were added so as to achieve a mass ratio of polymer (B3) to the total mass of the polymer (B3) and the zirconium oxide particles of 0.4. Thereafter, the zirconium oxide particles were dispersed with a ball mill until the average particle size in the suspension liquid reached 0.94 μm to thereby obtain the coating solution. HS-210 manufactured by NOF CORPORATION as a nonionic surfactant was added to the coating solution obtained, and the viscosity of the coating solution was adjusted to 9.3 mPa·s. Note that the viscosity of the coating solution was measured by the method mentioned above and the zirconium oxide used was one obtained by pulverizing raw stones.

This suspension liquid was applied to both the surfaces of the membrane main body by a spray method and dried to thereby obtain an ion exchange membrane having a coating layer containing the polymer (B3) and zirconium oxide particles.

The distribution density measured by the method mentioned above was 0.5 mg per 1 cm². Additionally, the coverage measured by the method mentioned above was 83.3-.

The ion exchange membrane was moistened so as to increase the weight by 2% by mass. Then, as a result of electrolysis performance evaluation on this ion exchange membrane, the voltage indicated was as low as 3.07 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as low as 0.51%/day, and high durability to impurities was indicated.

Example 2

An ion exchange membrane was prepared in the same manner as in Example 1 except that the amount of HS-210 used was reduced and the viscosity was changed to 12.0 mPa-s in Example 1. In this ion exchange membrane, the content of the fluorine-containing polymer in the binder was 100% by mass.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 56.5%.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as low as 3.08 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as low as 0.741/day, and high durability to impurities was indicated.

Example 3

An ion exchange membrane was prepared in the same manner as in Example 1 except that the amount of HS-210 used was raised and the viscosity was changed to 8.5 mPa-s in Example 1. In this ion exchange membrane, the content of the fluorine-containing polymer in the binder was 100 h by mass.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 92.9.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as low as 3.06 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as low as 0.42%/day, and high durability to impurities was indicated.

Example 4

An ion exchange membrane was prepared in the same manner as in Example 1 except that a suspension liquid having a mass ratio of the polymer (B3) to the total mass of the polymer (B3) and the zirconium oxide particles of 0.7 was used in Example 1. In this time, the amount of HS-210 used was equivalent to the amount in Example 1, and the viscosity of the coating solution was 11.0 mPa-s. In this ion exchange membrane, the content of the fluorine-containing polymer in the binder was 100% by mass.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 66.8-.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as low as 3.06 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as low as 0.27%/day, and high durability to impurities was indicated.

Example 5

An ion exchange membrane was prepared in the same manner as in Example 1 except that a suspension liquid having a mass ratio of the polymer (B3) to the total mass of the polymer (B3) and the zirconium oxide particles of 0.32 was used in Example 1. In this time, the amount of HS-210 used was equivalent to the amount in Example 1, and the viscosity of the coating solution was 8.6 mPa-s. In this ion exchange membrane, the content of the fluorine-containing polymer in the binder was 100% by mass.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 90.1-.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as low as 3.08 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as low as 0.61%/day, and high durability to impurities was indicated.

Comparative Example 1

An ion exchange membrane was prepared in the same manner as in Example 1 except that a suspension liquid having a mass ratio of the polymer (B3) to the total mass of the polymer (B3) and the zirconium oxide particles of 0.2 was used in Example 1. In this time, the amount of HS-210 used was equivalent to the amount in Example 1, and the viscosity of the coating solution was 8.6 mPa-s. In this ion exchange membrane, the content of the fluorine-containing polymer in the binder was 100% by mass.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 98.1-.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as high as 3.13 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as high as 1.01%/day and intensively influenced by the impurities.

Comparative Example 2

An ion exchange membrane was prepared in the same manner as in Example 1 except that a suspension liquid having a mass ratio of the polymer (B3) to the total mass of the polymer (B3) and the zirconium oxide particles of 0.2 was used and the viscosity was not adjusted with HS-210 in Example 1. In this case, the viscosity of the coating solution was 9.0 mPa-s, and the content of the fluorine-containing polymer in the binder was 100% by mass in this ion exchange membrane.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 97.7%.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as high as 3.14 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as high as 1.021/day and intensively influenced by the impurities.

Comparative Example 3

An ion exchange membrane was prepared in the same manner as in Example 1 except that the viscosity was not adjusted with HS-210 in Example 1. In this case, the viscosity of the coating solution was 14.0 mPa-s, and the content of the fluorine-containing polymer in the binder was 100% by mass in this ion exchange membrane.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 40.0%.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as high as 3.12 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as high as 0.91%/day and intensively influenced by the impurities.

Comparative Example 4

An ion exchange membrane was prepared in the same manner as in Example 1 except that a suspension liquid having a mass ratio of the polymer (B3) to the total mass of the polymer (B3) and the zirconium oxide particles of 0.6 was used and the viscosity was not adjusted with HS-210 in Example 1. In this case, the viscosity of the coating solution was 22.0 mPa-s, and the content of the fluorine-containing polymer in the binder was 100% by mass in this ion exchange membrane.

The distribution density was measured in the same manner as in Example 1, and the result was 0.5 mg per 1 cm². The coverage was measured in the same manner as in Example 1, and the result was 30.0%.

[Electrolysis Evaluation]

As a result of electrolysis performance evaluation on this ion exchange membrane under the same conditions as in Example 1, the voltage indicated was as high as 3.11 V. As a result of test on resistance to impurities, the decrease in the current efficiency was as high as 0.82%/day and intensively influenced by the impurities.

The results described above are all shown in Table 1 below.

TABLE 1 Example Example Example Example Example Comparative Comparative Cornparative Comparative 1 2 3 4 5 Example 1 Example 2 Example 3 Example 4 B3 ratio (B3/(B3 + ZrO2)) 0.4 0.4 0.4 0.7 0.32 0.2 0.2 0.4 0.6 Coating solution viscosity 9.3 12.0 8.5 11.0 8.6 8.6 9.0 14.0 22.0 (mPa · s) Coverage (%) 83.3 56.5 92.9 66.8 90.1 98.1 97.7 40.0 30.0 Distribution density 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 (mg/cm2) Voltage (V) 3.07 3.08 3.06 3.06 3.08 3.13 3.14 3.12 3.11 Reduction in current 0.51 0.74 0.42 0.27 0.61 1.01 1.02 0.91 0.82 efficiency (%/day)

REFERENCE SIGNS LIST

-   1 ion exchange membrane -   2 carboxylic acid layer -   3 sulfonic acid layer -   4 strengthening material -   membrane main body -   11 a, 11 b coating layer -   100 electrolyzer -   200 anode -   300 cathode 

1. An ion exchange membrane comprising: a membrane main body comprising a fluorine-containing polymer having an ion exchange group; and a coating layer arranged on at least one face of the membrane main body; wherein the coating layer comprises inorganic particles and a binder, a mass ratio of the binder to a total mass of the inorganic particles and the binder in the coating layer is 0.3 or more and 0.9 or less, and a coverage of the membrane main body with the coating layer is 50% or more.
 2. The ion exchange membrane according to claim 1, wherein the inorganic particles are particles comprising at least one mineral selected from the group consisting of oxides of Periodic Table Group IV elements, nitrides of Periodic Table Group IV elements, and carbides of Periodic Table Group IV elements.
 3. The ion exchange membrane according to claim 1, wherein the inorganic particles are particles of zirconium oxide.
 4. The ion exchange membrane according to claim 1, wherein the binder comprises a fluorine-containing polymer.
 5. The ion exchange membrane according to claim 1, wherein the binder comprises a fluorine-containing polymer having an ion exchange group derived from a carboxyl group or a sulfo group.
 6. A method for producing the ion exchange membrane according to claim 1, comprising: spraying a coating solution comprising inorganic particles, a binder, and a solvent by a spray system, followed by drying thereof to form a coating layer on a surface of the membrane main body, wherein the coating solution has a viscosity of 13 mPa·s or less.
 7. An electrolyzer comprising the ion exchange membrane according to claim
 1. 